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Great white sharks, grey skies and lightning were a setback for the thousands who flocked to Perth beaches in yesterday's warm weather.

Several beaches, including Quinns, Mullaloo, Sorrento and Hillarys, were closed in the afternoon because of lightning after a hot morning which saw the temperature hit 36C at 11.40am.

Surf Life Saving WA's volunteer duty inspector Simon O'Rourke said that the storm significantly reduced the number of beachgoers in the afternoon.

He said seven or eight swimmers were rescued at Scarborough and he urged people to heed any warning signs.

Mr O'Rourke said there had been significantly less drinking of alcohol on the beaches since a police operation this month.

On Saturday, police handed out 12 move-on notices, five liquor infringements and five liquor cautions at Scarborough. One person was charged by summons with drink-driving for having a blood-alcohol level over 0.08.

At other Perth beaches, swimmers such as Mullaloo surfer Frank Ward appeared undeterred after tagged great whites set off acoustic receivers at Ocean Reef and Garden Island north yesterday.

Another surfer reported being circled by a 2-2.5m shark as he paddled off Secret Harbour yesterday.

The Westpac Rescue Helicopter also saw a 3m shark about 200m north of Catherine Bay at Rottnest and boaties reported a great white near popular Thomson Bay.

Authorities believe the Rottnest sightings were probably the same shark.

Weather Bureau duty forecaster Luke Huntington said the warm weather was standard for this time of year with northerly winds bringing hot air from the interior.

A cooler change is expected today with 25C and a possible shower. There is a chance of thunderstorms on Wednesday and Thursday.